The present invention relates to optical projection using diffraction for uses such as three dimensional (3D) surface measurements for facial recognition or other purposes.
Optical projection of a pattern is used in applications such as 3D surface measurements. The positions of a pattern of dots caused by beams projected onto a flat surface can be determined. When the same pattern of dots is projected on a 3D surface to be measured, the positions of the dots will deviate from their designed positions as a result of the different intersection height on the 3D surface. These deviations can be measured and correlated to the different distances, or depth, of the 3D surface, and a 3D image can be generated.
One configuration uses a laser beam which is divided into multiple beams to generate the pattern of dots on the 3D surface, which are reflected back to a detector. FIG. 1 shows an example of a pattern of beams/dots, such as beam 101, projected onto a face 102. When a Diffractive Optical Element (DOE) is used to divide the incoming beam into N beams according the pattern design, the power of each diffracted beam (e.g., 1st order beam 101) is roughly equal to ηP/N where η is the diffraction efficiency and P is the incoming laser power. In most cases, the diffraction efficiency cannot reach 100 percent. The remaining power (1−η)P will remain in the un-diffracted beam (0th diffracted order), shown as beam 100, and scattering noise. The ratio of the power between the un-diffracted beam (0th diffracted order) to one of the diffracted order is equal to (1−η)N/η. For large N such as 1000 or more, even with 98% efficiency, each diffracted beam will have 0.098% of the incoming laser power and the 0th order has 2% of the incoming laser power. That means the 0th order has more than 20 times more laser power than the other diffracted orders. In order to detect the diffracted dot (101), a remote sensor may require the diffracted dot to have certain amount of laser power.
When used for facial detection, the laser power must be limited to avoid damaging the person's eyes. Even with non-visible IR lasers, too high a power of the laser can result in burning the retina. Because the 0th order 100 is 20 times higher, the laser power at the 0th order may exceed the minimum power for the safety of the eye.
US Published Patent Application No. 2011/0075259 described a technique to reduce the ratio of 0th order power to the diffracted order power by sub-dividing the DOE into M DOEs. Each sub DOE will have N/M dots. Now for each diffractive order in a sub DOE, the laser power of each diffractive beam is ηP1M/N. P1 is the laser power striking on each sub DOE. Under the best circumstances P1 for each sub DOE is equal to P/M. As a result the laser power of each diffracted beam is unchanged and equal to ηP/N. On the other hand the of 0th order power of each sub DOE is only (1−η)P/M. The ratio of the power between the un-diffracted beam (0th diffracted order) to one of the diffracted order is now equal to (1−η)N/Mη. Let's continue with the previous example and assume M is equal to 9. The 0th order beam to the diffracted beam ratio becomes 2.26. As a result, there is significantly less chance for 0th order to exceed the eye safety limit of laser power.